In all pressure butt welding processes high and consistent welding quality is required. This is particularly the case in the manufacture of chains as even a single badly welded chain link impairs the safety of the whole chain.
In conventional pressure butt welding machines the welding process, comprising in succession a heating and an upsetting phase, is controlled both kinematically and electrically by means of a central control shaft. Arranged on this control shaft are one or more cam discs whose outer contours control the operating sequence of the welding program. Thus, in the case of chain welding machines the advancing of the electrodes and of the upsetting dies or steels applying the longitudinal forces is generally controlled by a spring-loaded lever rolling over a cam disc, in combination with a planar gear. Synchronous therewith a further cam actuates the contacts switching the welding voltage on and off as the control shaft rotates.
This rigid control system has the serious disadvantage that there is no way of compensating for disturbing factors that adversely affect the quality of the welding. Such disturbing factors can include fluctuations in the power-supply voltage, variation in the impedance of the welding transformer as it heats up which, owing to the resulting change in the phase shift between the welding current and the welding voltage, results in changes in effective welding power, and the contact resistances between the welding electrodes and the workpiece which change from time to time. Other specific disturbing factors that impair the quality are those determined by the workpiece, for example the shape and material tolerances of the bent chain links. The last-mentioned condition has until now been countered by making very high demands on the bent chain links with regard to dimensional accuracy and constancy of the standards of quality of the material, which naturally make the production of such chains much more expensive.